Resistive touch screens are widely used in conventional CRTs and in flat-panel display devices in computers and in particular with portable computers. FIGS. 2 and 3 show a portion of a prior art four-wire resistive touch screen 10, which includes a transparent substrate 12, having a first conductive layer 14 defining a touch area. This conductive layer typically comprises indium tin oxide (ITO) or conductive polymers such as polythiophene. A flexible transparent cover sheet 16 includes a second conductive layer 18 that is physically separated from the first conductive layer 14 by spacer dots 20. Conductive patterns 30 having lower resistance than the conductive layer 14 defining an edge area are arranged over the conductive layer 14 at opposite edges of the conductive layer 14 on transparent substrate 12. The conductive patterns 30 are provided by an additional layer of material that is in electrical contact with conductive layer 14. Conductive patterns 30 are also provided in electrical contact with and at opposite edges of the conductive layer 18 on the flexible transparent cover sheet 16 (because of the four conductive patterns 30 this is commonly referred to as a four-wire design). The shape of these conductive patterns 30 can be adjusted to improve the linearity of the response of the touch screen. See for example U.S. Pat. No. 4,625,075 issued Nov. 25, 1986 to Jaeger. These conductive patterns 30 are used to provide electrical connection to the conductive layers 14 and 18.
In an alternative design (commonly called a five-wire design) all four conductive strips 30 are located on the substrate 12 and the second conductive layer 18 is the so-called fifth wire. The five-wire design may also utilize specially chosen patterns for the four conductors 30 on substrate 12 to improve the linearity of the device response.
The flexible transparent cover sheet 16 is deformed, for example by finger pressure, to cause the first and second conductive layers 14 and 18 to come into electrical contact. A voltage is applied across the conductive layers 14 via electrical connections 33 and a resulting signal is measured on the electrical connections 31 connected to layer 18 to determine the location of the touch in one direction. The voltage is then applied across the conductive layer 18 and the signal is measured on the electrical connection 33 to determine the location of the touch in the orthogonal direction. The conductive layers 14 and 18 have a resistance selected to optimize power usage and position sensing accuracy.
In conventional prior-art manufacturing processes, the conductors 30 are made of silver inks screen printed onto the conductive layers 14 and 18. In practice, this process has a number of disadvantages. First, the silver inks are costly and the screen printing process is expensive in that additional manufacturing steps and materials are needed. Second, unless they are carefully prepared and printed, the silver inks do not adhere well to the conductive layers. Moreover, the process of adhering the inks to the conductive layers may require high temperatures, creating problems for other materials in a touch screen or associated display system. Furthermore, the width of the edge area of the touch screen may need to be relatively large to accommodate the patterns used to linearize the response of the touch screen.
There is a need therefore for an improved means to provide conductive patterns for a resistive touch screen and a method of making the same that can reduce the width of the edge area, improve the robustness of the touch screen and reduce the cost of manufacture.